The present invention relates to medical anesthesia delivery systems for providing breathing gases and anesthesia to a patient. Specifically, the invention relates to a method and apparatus for operating a medical anesthesia machine and to an improved means of controlling the machine and determining faults in its operation.
Fundamentally, anesthesia machines are used during surgery by clinicians to deliver medical gases and inhaled anesthetic agent. Inspired breathing gases typically consist of a mixture of oxygen, nitrous oxide and other gases. The oxygen is supplemented to the patient to elevate the oxygen concentration above its concentration in air to provide a safe margin of inspired oxygen therapy. The anesthetic agent is added to the supply of breathing gases to provide the appropriate level of anesthesia so that the patient remains unconscious, sedated and relaxed during the surgical procedure. The inhaled anesthetic also provides amnesia of the surgical event.
It is common during the administration of anesthesia, for the patient to be connected to a partial rebreathing circuit, for example a circle breathing system. With such circuits, the patient's expired gases are recirculated, the CO.sub.2 in the exhaled gases scrubbed, and the resulting gases are replenished with fresh gas and again administered to the patient during inspiration.
A common technique used in anesthesia today is referred to as low flow anesthesia where a minimal amount of fresh gas is added to the system. The technique enhances warming and humidification of the gases in the patient circuit since less colder and dry fresh gas is added and, of course, there is a savings in the cost of the anesthetic agent since that agent is recirculated rather that being vented from the system to a scavenging system or the like.
Generally, fresh gas flows in the range of 0.5 liters per minute can be used, although low flow may be up to around three liters per minute. Basically, the fresh gas flow is less than the patients minute volume so that there is partial rebreathing and the lower the fresh gas flow, the more rebreathing occurs. Due to the recirculation gases, however, the actual anesthetic concentration set by the user to be delivered by the vaporizer as well as the oxygen concentration is different than the inspired anesthetic and oxygen concentrations since the gases circulating in the patient circuit, in effect, dilute the concentrations of the agent and the oxygen delivered in the fresh gas stream.
High flow anesthesia, on the other hand, may be in the order of providing fresh gas into the system at the rate of three liters per minute and above and less gas is recirculated from the patient's exhalation back through the system. The fresh gas flow approximates or exceeds the patient's minute volume such that the exhaled gases are vented out of the system. Typical high flows without gas recirculation occurs above 1.1 times the patient's minute volume and, as indicated, result in a higher usage of anesthetic agent as well as a loss of heat and humidification in the patient circuit.
In the anesthesia machines generally in use today, the gas and vapor delivery is mechanically actuated by the clinician who adjusts the desired flow of oxygen out to a common gas outlet. At high flow rates, where most of the inspired concentrations to the patient are from the fresh gas, the ratio of the flow settings approximates the delivery of inspired gases. Thus, at these high flows, the clinician can manually set the various flow settings to obtain the concentrations of gas and vapor delivered to the patient. As indicated, however, the use of high flow rates is wasteful of anesthetic agent etc. as most of the agent laden gas is vented from the anesthesia machine.
The move to low flow anesthesia, while beneficial from an efficiency standpoint, is, however, tedious for the clinician to carry out manual adjustments of flow to achieve the targeted inspired concentration to the patient.
Accordingly, a new generation of anesthesia machines has emerged to facilitate very low to closed circuit anesthesia. Examples of these new low flow anesthesia machines are the Physioflex machine by Physio, Inc. and the machine described in U.S. Pat. No. 5,094,235 of Westenskow. With these newer machines, the user sets the oxygen concentration and either the inspired or expired anesthetic agent concentration to be delivered to the patient instead of setting fresh gas flow rate of agent vapor concentration delivered out of a common gas outlet. The machines use electronically controlled valves to blend the gas mixture and an electronically controlled vaporizer to deliver the anesthetic vapor. These electronic devices are controlled by a central processing unit to achieve the precise control needed at the low flows.
A patient respiratory gas monitor located at the common Y-piece of the breathing circuit senses and analyzes the oxygen concentration and the agent concentration in the inspired and expired gases to and/or from the patient. Another agent and oxygen monitor, collectively referred to as an inspiratory gas monitor, is used to monitor the agent and oxygen concentrations of gases in the inspired limb of the patient circuit and that measurement signal is fed to a CPU to close the control loops that actuate the electronic gas mixer and the electronic anesthetic vaporizer to achieve the clinician's desired delivery settings that are inputted to the CPU. The gas monitor in the Y-piece can thus check the feedback from the gas monitor used in the automatic control scheme and, preferably, the monitoring and feedback monitors are of differing technologies to avoid unrecognizable common mode failures.
Thus it would be advantageous to quickly and accurately determine when the monitoring and feedback monitors are malfunctioning totally or are simply out of calibration so that proper and prompt corrective action can be taken.
As with all monitors, their measurements drift and change with different environmental conditions. Differences in sensor technologies, location of measurements and processing technique further exacerbate any mismatch in the measurements of the same parameters. To overcome this, a bias correction can reduce the disagreement between these measurements. Furthermore, if the monitored data are consistent but different from the user set, it can be inferred that the delivery device failed to deliver the user set concentrations. Accordingly, it would be advantageous to be able to carry out that procedure automatically.